This invention relates generally to stripping optical fibers, and in particular to a method and apparatus for rapidly and efficiently stripping optical fibers without using chemicals and without reducing the tensile strength of the fiber.
Fiber optic cables are widely used in modern optical devices and optical communications systems. Optical fibers are usually coated with a protective layer, for example a polymer coating, in order to protect the surface of the fiber from chemical or mechanical damage. It is necessary to remove the protective coating in order to prepare the fibers to be cleaved and spliced, or in order to further process the fibers to manufacture optical devices such as optical sensors and other optical communications network components.
Conventional stripping methods include mechanical stripping, chemical stripping, and thermal stripping. These methods all suffer from a number of defects. Mechanical stripping typically involves a stripping tool, similar to a wire stripper, which cuts through the coating and scrapes it off. A major disadvantage is that mechanical stripping typically nicks or scratches the glass fiber surface, eventually leading to cracks and to a degradation in the tensile strength of the fiber. By way of example, the tensile strength of an optical fiber may be reduced from about 15-16 pounds before mechanical stripping to about 3-5 pounds after mechanical stripping. The optical fiber""s longevity is thereby reduced.
Chemical stripping uses solvents or concentrated acids to remove the polymer coating. In the prior art, acid stripping is often performed using a sulfuric nitric mixture that includes about 95% sulfuric acid and about 5% nitric acid. While this prior art method reduces tensile strength degradation, an acid residue may typically be left on the fiber surface at the splice point. Therefore, using chemical stripping on titanium dioxide color coded fiber degrades the splice strength. Also, chemical stripping as performed in the prior art is very costly. Finally, there are major safety concerns inherent in chemical stripping methods. Ventilation and safety equipment may be needed when using acids for the stripping process. Human operators performing acid stripping require facilities having well-ventilated areas, preferably with exhaust or ventilation hoods for removing acid fumes. They may also require protective gear, such as protective clothing and gloves for avoiding acid burns, and protective breathing apparatus for protection from acid fumes in the air. Storing, handling, and transporting the acids are also extremely hazardous.
Thermal stripping processes use heat to remove the coating. In particular, hot air stripping methods have been used in the prior art, in which heat (e.g., at about 470xc2x0 C.) is applied to the polymer coating, causing the polymer coating to heat to a break temperature, at which point the removal of the coating begins. Some prior art hot air stripping methods, such as disclosed for example in U.S. Pat. No. 5,968,283, involve translation of the fiber optical cable. The fiber optical cable is moved over the heat source so that heat directly from the heat source is applied along the optical fiber cable between selected points, causing the corresponding polymer coating to curl and drop off the optical fiber.
These hot air stripping methods suffer from a number of disadvantages. For example, polymer coating curls can remain attached to the fiber optical cable. To prevent the polymer coatings from remaining attached to the optical fiber, it may be necessary to split the polymer coating from the optical fiber at two points, before attempting to curl a section of the polymer coating off the optical fiber. Finally, these prior art methods tend to expose the heated air stream to carbon or oxidizing metals from the heat source, so that particles of carbon or oxidizing metals are deposited on the fiber during the heating process. When such unwanted particles are deposited on the fiber, the tensile strength and performance characteristics of the fiber may be compromised.
Another disadvantage of methods such as the method disclosed in U.S. Pat. No. 5,968,283 is that these methods use a hot air heat source that must generate heat at the break temperature, before starting to heat the polymer coating. This usually requires a flow of hot air for a period of time, before each stripping process begins. Thus, there tends to be a relatively long ramp up time. Devices such as heat shrink guns rated at 1500 Watts, which generate forced air at a temperature of about 470 degrees Celsius, are thus used as the heat source in these prior art methods. When splicing cycles are repeated, the flow of very hot air may be continuous. A continuous flow of very hot air can make it extremely hot and dangerous for the operator and cause a great deal of wasted energy.
It is an object of this invention to provide a method and apparatus for stripping fiber optical cable that do not suffer from the disadvantages described above. In particular, it is an object of this invention to provide a method and apparatus for stripping fiber optical cable without using chemicals, and without reducing the tensile strength of the fiber. It is another object of this invention to provide a method and apparatus for stripping fiber without curling the polymer coating. It is another object of this invention to provide a method and apparatus for stripping fiber more rapidly and efficiently, as compared to prior art methods, and without leaving any coating residues on the fiber. It is yet another object of this invention to provide a method and apparatus for stripping fiber that can be used to strip titanium dioxide color coded fiber, without degrading the splice strength of the fiber. It is yet another object of the present invention to provide and method and apparatus for stripping optical fiber that does not require a continuous flow of hot air. It is another object of the present invention to provide a method and apparatus for translating the stripper or portions thereof, the fiber or some combination thereof
The present invention provides a system and method for heat stripping an optical fiber (e.g., titanium dioxide color coded fiber). A short, heated burst of air is injected from a forced air heat source, and applied to one or more portions of the optical fiber. A short burst of air lasts less than about one second, and has a temperature of about 700-1100 degrees C. This is useful in quickly stripping a portion of the fiber cable (or spot stripping). The stripper may be a translatable stripper, whereby the stripper or portions thereof, the fiber(s), or some combination thereof, are translatable. In such a case, prolonged or multi-burst techniques may be used to strip one or more extended lengths of one or more fiber optic cables. In either case, due to the high temperature, the outer coating of the optical fiber is immediately removed, without degrading the original tensile strength of the fiber. No coating residue remains on the fiber, and no curling of the coating occurs. While heated air is used in a preferred embodiment of the invention, other embodiments may use other substances, such as other gases and fluids.
A system for stripping an optical fiber in accordance with the present invention includes an air source and means for generating short bursts or streams of air from the air source, by releasing compressed air during short periods of time. Typically, each short burst of air lasts less than one second. However, for stripping extended lengths of fiber the burst of air may have a longer duration, e.g., 4-5 seconds.
In one embodiment of the invention, the means for generating bursts of air includes an air pressure generator for creating air pressure, an air pressure controller for controlling air pressure, and an air flow regulator for regulating the flow of air out of the means for generating bursts of air, so as to controllably release compressed air from the means for generating bursts of air during very short time intervals. In one form of the invention, the air flow regulator may be a solenoid valve controlled by a timer.
The optical fiber stripping system further includes a heater for heating the bursts of air to a temperature sufficient to remove the outer coating from the optical fiber with a single burst. Typically, the requisite temperature is from about 700 degrees Celsius to about 1100 degrees Celsius. The heater heats the air bursts without bringing the air into contact with the heat source of the heater. In this way, the air avoids exposure to unwanted contaminating particles from the heat source, such as carbon or oxidized particles. The unwanted particles are thus prevented from being deposited on the fiber, and from reducing the tensile strength or performance characteristics of the fiber. The heater can be used to efficiently heat substances other than air, such as other gases and fluids.
The heater includes a heater core having a heat generating element. The heater core supplies heat to a heat chamber. An air conduit receives air from the means for generating bursts of air and is preferably configured to also receive heat from the heater core, thereby preheating the air. Along with a heat chamber outlet port, the air conduit and heat chamber form an isolated air transport path. When air is injected from the means for generating bursts of air into the air conduit, heat generated by the heat generating element in the heater core is transferred to the air while the burst of air flows through the conduit and through the heat chamber. In this way, the air stream is heated to a temperature sufficient to strip an optical fiber, while remaining isolated from the heat generating element in the heater core. An air output nozzle connected to the outlet port of the heat chamber directs the heated burst of air at the portion of the optical fiber to be stripped. The outer coating of the fiber is vaporized and removed almost instantly. In other forms, preheating in an air conduit may not be provided.
In various embodiments, the stripper or portions thereof are translatable with respect to the fiber. In other embodiments, the fiber may be translatable with respect to the stripper, or portions thereof. In such translatable strippers, multiple bursts of air may be used to strip an extended length of fiber, different areas on the same fiber, multiple fibers using the same output nozzle, or some combination thereof. Otherwise, several output nozzles may be provided, each configured for alignment with different fibers or different areas of the same fiber, and the heat chamber outlet port may be translatable (or include a translatable extension member) such that the outlet port couples to each of several output nozzles. Otherwise, multiple outlet ports and output nozzles may be provided, and one or more of those may be translatable.
The present invention features a method for stripping one or more optical fibers. The method includes delivering bursts, i.e., each burst of air characterized by a relatively short duration in time. The air bursts are injected into a heater via an isolated air transport path. The heater includes a heat chamber and a heat generating element. The bursts of air are heated within the heat chamber to a temperature sufficient to vaporize the outer coating from the fiber, without the air being exposed to the heat generating element. In one form, a single short burst of air of about 1 second or less is directed at a portion of the optical fiber to be stripped, so as to thermally remove the outer coating from the optical fiber within less than one second, i.e., spot stripping. In another form, continuous stripping is used to strip an extended portion of a fiber. Continuous stripping may be accomplished using a multi-burst technique where a series of closely spaced short bursts are applied to the extended portion of the fiber. In another form, continuous stripping is accomplished by a prolonged burst technique where a burst of about 4-5 seconds, as an example only, is applied along a length of fiber to be stripped. The actual duration of the prolonged burst is determined as a function of the length of the portion of the fiber to be stripped. Spot stripping or continuous stripping may be used with a single portion of a single fiber, different portions of the same fiber, or on different fibers. In various embodiments of the method, the output nozzle or stripper is translatable, the fiber or fibers are translatable, or some combination thereof.